1. Field of the Invention
This invention relates to a process for catalytically effecting the stereochemical isomerization of tetrahydrodicyclopentadiene.
2. Description of the Prior Art
A hydrocarbon fuel for jet propelled, limited-volume systems; e.g., missiles, must exhibit a requisite combination of properties in order to be useful for this purpose. Foremost, the fuel should possess a high density and a corresponding high heat of combustion. Additionally it is required that such high energy fuels have a low freezing point and exceptional chemical stability.
A hydrocarbon composition meeting the foregoing requirements represents a complex chemical structure which, in general, must be paintakenly synthesized. A high energy fuel of this type is exemplified by exo-tetrahydrodicyclopentadiene (exo-THDCPD) which has been adopted by the U.S. Air Force for certain jet propelled systems and accordingly designated JP-10. The procedure involved for producing JP-10 consists of first dimerizing cyclopentadiene followed by completely hydrogenating the resultant dimer to provide the endo stereo isomeric form of the tetrahydro derivative.
Endo-THDCPD is unsuitable as a fuel per se because of the compound's high melting point; i.e., 77.degree. C. Accordingly, to obtain JP-10, the endo isomeric precursor is converted to its exo form having a freezing point of about -79.degree. C. This isomerization step represents the most critical part of the overall synthesis procedure in that it is comparatively fraught with difficulties. In U.S. Pat. No. 3,381,046 it is taught that the indicated isomerization can be effected through the agency of a strong acid catalyst. Representative of such catalysts include strong Bronsted and Lewis acids. The Bronsted acids, such as the preferred acid; viz.; sulfuric acid, suffer in that the resultant conversion yields are economically unacceptable for commercial production. The use of a strong Lewis acid, on the other hand, is prone to cause the isomerization reaction to proceed beyond the exo isomer thereby resulting in the objectionable formation of substantial amounts of transdecalin and adamantane.
Subsequent workers in this field have proposed that aluminum chloride can be effectively utilized as an isomerization catalyst while minimizing the difficulties referred to in the above-mentioned prior art provided the isomerization reaction temperature is carefully controlled. U.S. Pat. No. 4,086,284 is exemplary of this prior art. The latter teaches that the isomerization temperature is not to exceed 90.degree. C. and more preferably is in the order of about 70.degree. C. in order to avoid the formation of transdecalin and adamantane and to prevent an uncontrolled exotherm from occurring. The foremost disadvantage of this method of operation resides in the commercially intolerable conversion rates that are applicable. Moreover, this disadvantage is exacerbated by the fact that the presence of an inert solvent is practically necessitated because of physical handling difficulties encountered in carrying out the reaction at the relatively low temperatures specified.